Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to efficiently move the head between tracks and to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline.
In a disk file digital servo control system, a microprocessor utilizes a control signal algorithm to calculate a digital control signal based upon the digital values of certain state variables such as PES, VCM current and head velocity. The digital control signal is converted to an analog signal and amplified to provide input current to the VCM. Such a digital servo control system is described in U.S. Pat. No. 4,412,161, wherein the digital control signal is calculated recursively from prior control signals and prior values of the PES.
A recent development in digital disk file servo control systems, as described in assignee's U.S. Pat. No. 4,679,103, is a system which, as part of the computation of the control signal to the actuator, makes use of a state estimator algorithm to estimate the state (i.e., position, velocity and acceleration) of the head. In this type of system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES and the VCM input current, and computes, through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal to provide a power amplifier control signal. A power amplifier then generates a new VCM input current. The method of estimating the state of the physical plant to be controlled in such a digital servo control system requires the use of estimator constants, the derivation of which is described in Digital Control of Dynamic Systems, Franklin and Powell, Addison-Wesley Publishing Co. (1983), chapter 6, pages 131-139. In the case of a disk file, these estimator constants are dependent upon the values of certain physical parameters of the disk file, such as the moving mass of the coil and head/arm assembly, the VCM force factor (the force applied to the coil per unit of input current), the gain of the VCM power amplifier, the PES gain and the time between PES samples (the PES sampling time).
In prior art digital servo control systems, the analog VCM current is measured and converted to digital form by an analog-to-digital (A/D) converter. This measured VCM current is required for two reasons. First, during track following or short seeks, when the power amplifier is not in saturation, the measured current is needed to provide closed-loop control. Secondly, during long seeks when the power amplifier is saturated, the digital value of the coil current is needed so that the state estimator can accurately predict head velocity. It is thus desirable in disk file digital servo control systems which use a state estimator to eliminate the hardware cost and time delay associated with A/D conversion, while retaining accurate VCM current information.